Banks are finding competition for bank-like services from some surprising sectors.

That's because, thanks to changing technology and customer preferences, traditional banks could see a large percentage of their market share up for grabs. According to a study from Accenture, 35 percent of the market share for North American banks is at stake by 2020. As much as 15 percent of bank revenue could shift to online-only banking. The other 20 percent could go to retailers.

Take Starbucks, for example. As Wired shows, Starbucks, with its pre-paid cards accounting for $2.5 billion in U.S. sales last year, has services that look increasingly bank-like.

To be sure, those billions don’t amount to that much in the broader context of traditional banking. Still, the cards represent two traditional banking functions now being handled by Starbucks — payments and storing money. If you buy a friend a Starbucks card with your own Starbucks card, you’ve added a third function: money transfers. In a sense, you’ve taken money you’ve saved with Starbucks and used it to pay someone else while Starbucks brokers the transaction.

Then there's Google which is rolling out a physical debit card that is linked to users' Google Wallet account.

But it's not just large companies. Here's a good list of some of the most innovative companies, startups and products that are hoping to cut into banking revenues.

Banks aren't just giving up revenue quietly, however. They argue that some of these services from non-banking companies, especially Walmart, need greater federal oversight.

It's not all bad for traditional banks. As Wayne Busch and Juan Pedro Moreno of Accenture point out, banks still have a lot of customers and access to transaction data, but they'll need to innovate to keep from losing out on revenue to retailers and tech companies.

As Busch and Moreno say at Harvard Business Review: "Instead of simply enabling customers to save money and pay for things, banks have the potential to combine their vast transaction data with new digital tools to help customers make decisions on what to buy, and where and when to buy it – whether it’s dinner and a movie or a new home."